Variable electric filter



July 25, 1939. f-R. RECHNI TZER 2,157,452

'1 v VARIABLE ELECTRIC FILTER Filed March 4, i957 .ZZig. 2

INVENTOR I RUDOLF REGHNITZER ATTORNEY 2,167,462 PATENT OFFICE VARIABLEELECTRIC FILTER- Rudolf Rechnitzer, Budapest, Hungary, assignor toTelefunken Gesellschaft fiir Drahtlose Telegraphie m. b. H., Berlin,Germany, a corporation of Germany Application March 4, 1937, Serial No.128,959 In Germany March 5, 1936 9 Claims.

The present invention is concerned with a variable electric filter whichcomprises ohmic resistances and reactances, and which is primarilyintended for filters containing a plurality of units or meshes anddesigned to insure tone or timbre control in receiver sets andamplifiers. According to the invention, the filter incorporates aresistance device whose ohmic resistance is a function of the currentflowing therethrough and which serves to regulate the limiting or cutofffrequency.

In the case of multi-mesh filters the present invention oifers theadvantage that for the actuation of a single resistance serving forchanging the current all of the resistances included in the filter arevaried. In the case of a single-unit filter the present invention offersthe advantage that changes in current caused by the resistance changecausing the same could, for instance, be used at the same time forvolume control purposes. By the operation of a single resistance, it isthus possible to insure a faithful regulation of the acoustic volumepleasing to the ear. In other words, in accordance with the presentinvention during passages of low volume intensity, the medium-pitchedand'the high-pitched notes are weakened more markedly than the lowpitched notes.

In the drawing accompanying the specification, Figure 1 illustratesschematically the invention incorporating a circuit in which regulationof the tone is provided by a low pass filter; Figure 2 illustrates theinvention applied to a circuit utilizing a series type of filter; Figure3 is a diagrammatic representation of the circuit incorporating theinvention and utilizing the filter in the plate circuit of a tube;Figure 4 illustrates the invention in connection with a resistancecoupling arrangement; and, Figure 5 illus- 40 trates the invention in acircuit wherein volume control is combined with tone control.

Fig. 1 shows a .circuit organization in which regulation of the timbreor tone control is made feasible by the agency of low-pass filter I, 2.For

45 this object the amount of direct current flowing through resistance Iis adjusted by the aid of the potentiometer 4. As the direct currentflow becomes smaller, the resistance of the various meshes of the filtergrows, with the result that the limiting or cutoff frequency of thefilter is shifted in the direction of the lower frequencies. While inthe circuit scheme shown in Fig. 1 the filter is so connected that thecharacteristic falls slowly towards the high frequencies, the 55 filtershown in Fig. 2 is of the "line (series) type so that the characteristicis caused to droop suddenly in the presence of a certain frequency. Inthe arrangement shown in Figure 2, auxiliary resistances 5 and 6 areprovided in order to shut 60 off the regulator current for the filt r.

Also in this arrangement the cutofl frequency, when the intensity of thecurrent is low, will be shifted in the direction of the lowerfrequencies. In the arrangement shown in Fig. 3 the filter is includedin the plate circuit of the tube. This arrangement is less satisfactorythan some of the other arrangements described herein for the reason thata change in the plate potential is occasioned at the same time. Fig. 4illustrates another embodiment which resembles the scheme shown in Fig.1, though it is here used in connection with resistance coupling. Inorder that the change in the value of resistance I caused by changes ofcurrent may not be conducive to a change in the gain, the lowest valueof the resistance I is preferably made higher than that of resistance 8.

Fig. 5 shows a circuit organization in which volume control is combinedwith timbre or tonecontrol. When the volume is comparatively low, asmaller part of the potentiometer is shortcircuited for D. C. by theprimary winding, with the result that the current flowing across theresistances I decreases. Thevalue of the resistances I becomes greateras a consequence so that the higher pitched notes will be attenuatedmore markedly.

It will be understood that there are many ways of incorporating thebasic idea of this invention such as in connection with filterscontaining, for instance, coils in lieu of condensers.

What I claim is:

1. In an audio frequency transmission system including means to connectthe same to a source of audio frequency voltage and an audio frequencyutilizing load, an electrical network for regulating the audio frequencytransmission between the source and the load, said network including afilter unit provided with a resistance element the alternating currentresistance of which is a function of the direct current component ofcurrent passing through it, a source of direct current connected acrosssaid last. named resistance element and means for controlling the amountof current passing from said source through the last named resistanceelement.

2. In an audio frequency transmission system, means for connecting thesystem to a source of audio frequency voltage, an electronic tube havingan anode, a cathode and an input grid electrode, a source of anodecurrent for said tube,

said source having a positive terminal and a negative terminal, aconnection including a resistance element between the anode and thepositive terminal of the source and means connecting the negativeterminal and the cathode, a connection between the anode of the tube andthe cathode thereof and including a condenser and at least tworesistance elements in series, means for connecting said two resistanceelements in series across said source of direct current whereby a flowof current is produced through said resistance elements, one of saidresistance elements having an alternating current resistance which is afunction of the direct current component of current passing therethroughand means for varying the effective value of the said other seriesresistance which is in series between the first of said two seriesresistances and the source of direct current to thereby control the flowof direct current through the first of said series resistance elements,a utilizing circuit and means for connecting the utilizing circuitacross the second of said series resistance.

3. The combination with an audiofrequency amplifier of a tone controlcircuit therefor including a filter network provided with means forgiving to the filter network variable frequency characteristics, saidmea including a resistance device having an alternating current re-'sistance which is a function of the direct current component of currentpassing through it, a source of direct current, means connecting saidresistance device across said source, said .last

named means including variable means for controlling the amount ofcurrent flowing from the source through the resistance device.

4. In a tone compensated volume control device for an audio frequencyamplifier including a source of audio frequency voltage and an audiofrequency utilizing load, a network connected between the source and theutilizing load, said network including a resistance having analternating current resistance which is a function of direct currentcomponent of current caused to flow through it and acting to vary thefrequency characteristics of the filter circuit and means included inthe network for controlling the amount of audio frequency currenttransferred from the source to the utilizing load, operation of the lastnamed means also acting to vary the direct current component of currentflowing through the filter network.

5. In an audio frequency amplifier, an' electronic tube provided with aninput circuit and an output circuit, means for connecting the inputcircuit to a source of audio frequency voltage, an audio frequencyutilizing load, a source of anode direct current for said tube, saidsource being connected in the output circuit thereof, a filter networkin said output circuit for regulating the audio frequency transmissionthrough the output circuit, said filter network including a resistanceelement having an alternating current resistance which is a function ofdirect current component of current passing therethrough, meansincluding a resistance device for connecting said first named resistanceelement across said source of anode current, said resistance devicebeing effectively shunted across the output circuit of said electronictube, said resistance device having a variable tap, means for connectingthe audio frequency utilizing load between one end of the last namedresistance device and said variable tap to thereby provide for variabletransfer of energy from the output of said tube to the utilizing load,and a path of low direct current resistance connected between saidvariable tap and said end of the last named resistance wherebyvariations of the variable tap along said last named resistance vary theeffective resistanceconnected in series between said source and saidresistance element to thereby vary the fiow of current through the firstnamed resistance element in accordance with variations in the amount ofaudio frequency energy transferred from the output of the tube to theaudio frequency utilizing circuit.

6. The combination with an audio frequency amplifier of a tone controlcircuit therefor, including a filter network provided with means forgiving'to the filter network variable frequency characteristics, saidmeans including an ohmic resistance device having an alternating currentresistance which is a function of: the direct current component ofcurrent passing through it, a source of direct current, means connectingsaid resistance device across said source of current, an audio frequencyutilizing load, means for connecting the load to said audio frequencyamplifier, said last named means including a variable device adapted tocontrol the amount of energy transferred from the audio frequencyamplifier,

.said' variable device also acting upon operation to vary the amount ofcurrent transferred from said source of direct current to said ohmicresistance device whereby the frequency characteristics of said filternetwork are varied simultaneously with variations in the energytransferred between the audio frequency amplifier and the utilizingload,

7. In an audio frequency transmission system having input terminals forconnection to a source of audio frequency voltage and output terminalsfor connection to an audio frequency utilizing device, anetworkincluding a filter unit pro vided with resistance means thealternating current resistance of which is a function of the directcurrent component of current passing therethrough, said network having afrequency characteristic depending upon the magnitude of the alternatingcurrent resistance of said resistance means, a source of direct currentand means including a variable resistance device for connecting saidresistance means across said source of direct current whereby currentfrom said last named source flowing through said resistance means may becontrolled to thereby vary the frequency characteristic of the network.

8. A transmission network including a plurality of resistance elementswhose alternating current resistance depends upon the direct currentcomponent of current flowing therethrough, said network having afrequency discriminating characteristic dependent upon the alternatingcurrent resistance of said resistance elements, means for providing aflow of direct, current through said resistance elements and means foradjusting the magnitude of said direct current flow through saidresistance elements to thereby control the frequency characteristic ofsaid network.

9. In a transmission network for coupling a source of energy to autilizing circuit, a filter circuit having a resistance element whosealternating current resistance depends upon the direct currentcomponents of current passing therethrough, said filter circuit having afrequency discriminating characteristic whichis dependent upon thealternating current resistance of said resistance element, means forproviding direct current flow through said resistance element and meansfor adjusting the magnitude of the direct current flow through saidresistance element to thereby control the frequency characteristic ofsaid filter circuit, said last named means also acting to control theamount of energy transferred from said source to said utilizing circuitthrough the transmission network,

RUDOLF RECHNITZER.

